You're more likely to find them at Chinese shops. They keep them on the tea tray, and pour some excess over the animal for luck. I got mine (my avatar) on eBay, but our local tea shop (caters to Chinese, it's in an all-Asian shopping center) has a couple as well. I know that some do use small jade or wood carvings as well, so the yixing animals aren't the only ones you can use.

I contacted Yunnan Sourcing on eBay about them and he wrote back saying he's got access to "hundreds of yixing beasts". You can probably email him your request and see if he can get you what you want. I'm working on a list for him now, if you'd like me to make the inquiry just PM me.